Patient performance with the Cochlear Corporation "20 + 2" implant: bipolar versus monopolar activation

A within-subjects comparison of monopolar versus bipolar stimulation was performed using a modified version of the Nucleus mini-22 cochlear implant, the Nucleus "20 + 2" implant. Six subjects underwent implantation with this device, which is identical to the Nucleus 22 cochlear implant with the addition of two extracochlear indifferent electrodes. These electrodes provide two monopolar modes of stimulation in addition to the standard bipolar modes used with the Nucleus 22 device. One of the indifferent electrodes is a ball placed under the temporalis muscle (MP1), whereas the second electrode is mounted on the lateral aspect of the receiver-stimulator (MP2). After a pre-experimental phase, subjects used each of three stimulation modes (BP + 1, MP1, and MP2) for a total of 4 weeks each. Variables tested with each mode included electric thresholds, comfort levels, dynamic ranges, and speech recognition. Both the MP1 and MP2 modes of stimulation required significantly less current than the bipolar mode (BP + 1) to reach threshold and comfort level. Analysis of dynamic range data also indicated a significant stimulation mode effect. However, stimulation in the various modes did not significantly affect speech recognition scores, although two subjects demonstrated significantly improved speech recognition scores when programmed in a monopolar mode.